The present invention relates to devices for determining the concentration of fine particles suspended in air or another fluid, and more particularly to optical single particle detectors.
A wide variety of instruments are available for measuring the size and concentration of airborne particulates, for purposes as divergent as monitoring air pollution, measuring the effectiveness of filters in ventilation systems and elsewhere, and determining particle concentrations in liquids. Optical systems are frequently employed in these situations because they enable non-intrusive particle measurement and provide real time data. Such devices include photometers, which measure the total light scattered from many particles at once, and single particle detectors.
Single particle detectors provide more precise information on particle concentration and size distribution, and are superior to photometers in measuring low concentrations. Such instruments detect particles either by light scattering or by light extinction. Light scattering involves measuring light that a particle refracts, diffracts or re-radiates, at an angle or range of angles different than the original direction of the light. The extinction method involves continuously measuring light from a source to a detector. When a particle passes through the light, it decreases the amount of light reaching the detector, with larger particles diminishing light to a greater degree.
Whether they rely on light scattering or extinction, single particle detectors are complex and expensive. In both cases, precision optics are necessary to create a tiny, high intensity light spot. This maximizes the amount of light scattered from or blocked by a single particle. Extinction systems sometimes pass particles through the resonant cavity of a gas laser, to provide a higher intensity light beam.
In either type of system, knowing or controlling the volume of air containing the aerosol is essential for an accurate measurement of concentration. Generally this is done with a sampling pump, which draws air through the light beam at a constant rate.
While presently available single particle detectors are suitable in a wide variety of applications, they are not well suited to certain needs in industry. Most of the current technology is geared towards detecting the smallest particle possible. For example, earth moving vehicles and other construction equipment frequently are subjected to harsh environments involving dust, temperature extremes, vibration and shock. The internal combustion engines running this equipment can be destroyed in minutes by unduly high particulate concentrations in the engine intake air. Thus, the ability to provide an early warning of potentially damaging aerosol concentrations is critical.
Currently available measurement approaches, however, are not suited to this end. Particulate concentrations downstream of a properly functioning air filter are substantially lower than concentrations appropriate for photometers. Yet single particle optical systems require precision optics with several lenses. Components of the device must be aligned, and usually each completed device must be individually calibrated due to the unique shape of its optical particle sensing volume. The relatively large size of certain optical system components generally prevent the system from being installed directly in the fluid flow. The sampling system, needed to draw aerosol into the sensing volume, adds to the cost of the instrument. Sampling decreases accuracy, because particles are lost to the sampling tube walls as the aerosol is drawn by the sampling pump. These optical sensing systems lack sufficient structural rigidity for installation in hostile environments involving shock, vibration and temperature extremes.
Therefore, it is an object of the present invention to provide a single particle detecting device with a small in-situ probe that can be mounted directly in an aerosol flow.
Another object is to provide a single particle detecting device which requires no lenses, mirrors or other precision optics, and further requires no alignment or calibration, yet provides a well defined, repeatable particle sensing volume.
A further object is to provide a single particle detection device which is sufficiently rugged to perform reliably in harsh environments.
Yet another object of the invention is to provide a single particle detection device for measuring concentrations independently of the velocity of the air stream in which the particles are suspended.